This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners.
Modern design of high power microwave amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment. The microwave tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (FIG. 1); this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers”, popularly called elsewhere also “tuners” because of the included tuning circuits.
Impedance tuners consist, in general, of a transmission line and a number of serial or parallel elements, fixed or adjustable, which create a variable reactance, allowing thus the synthesis of various impedances (or reflection factors) covering parts or the totality of the Smith chart (the normalized reflection factor area).
The relation between reflection factor and impedance is given by Γ=(Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX and Zo is the characteristic impedance of the transmission line itself.
A number of techniques have been used in the past to manufacture such tuners; one technique is the “slide-screw” tuner (FIGS. 2 and 3) consisting of a slotted transmission airline (1, 2) in which a plunger (or probe, or slug) (3) can be inserted vertically, either manually or remotely, and creates a variable capacitive load and thus a reflection vector at any given frequency. Moving the probe (3) closer (4) to the central conductor (2) increases the capacitive load and thus the magnitude of the reflection factor. Moving the probe along the airline (1) changes the phase of the reflection factor. For a 360 degrees phase change a horizontal travel distance of one half of a wavelength (λ/2) is required, which determines the total length of the tuner. For such a tuner to operate at 1 GHz a total free travel length of 150 mm is needed, for 100 MHz this length is 1,500 mm (or 1.5 meters). For lower frequencies the required length and thus the size of the tuner becomes even un-handier (3 meters for 50 MHz, 4.5 meters for 33 MHz and so on.) and practically impossible to manufacture in one piece. In view of the required precision of such a tuner these sizes are prohibitive.
One major advantage of slide-screw tuners is the fact that the capacitive loads (3) used have “low-pass” behavior, meaning that at low frequencies they are transparent and do not create any noticeable reflection factor. Typical transistors having higher gain at low frequencies have the tendency to generate spurious oscillations when presented with high reflection factors at these frequencies. Slide screw tuners are therefore the best impedance tuners for testing such oscillation-prone devices under test (DUT's).
Another technique used is the “multi-stub” tuner (FIG. 4). In this case a number of typically three shorted parallel stubs (5, 6, 7) are used connected to a coaxial transmission airline (8) at fixed distances (9, 10). Changing the position of the shorts (5, 6, 7) inside each parallel stub, manually or remotely, creates a variable reactance, which can be capacitive or inductive, depending on the length of the stub, and creates an adjustable reflection factor at the test port of the tuner. Again, the distance (9,10) between the stubs being fixed, the frequency coverage of these tuners is limited; the reason being that, for optimum coverage of the Smith chart, the optimum distance between the stubs must be 120° in reflection (or one sixth of a wavelength, λ/6). This being the case some manufacturers have used two different lengths (9,10) between stubs, in order to somehow widen the frequency range (FIG. 4). Nevertheless the fact remains that, for optimum coverage, a three stub tuner must be at least 2×λ/6=λ/3 long, in which case all estimates of the previous paragraph apply. At low frequencies the unit becomes big, un-handy and difficult to manufacture; in addition to the fact, that its frequency coverage remains limited and non-adjustable.
A further disadvantage of the multi-stub tuners is the fact that the variable shorts (5,6,7) do not allow DC bias to flow through the tuner in order to bias the DUT; this creates additional DC supply requirements for the test fixture, in which the DUT is embedded (FIG. 1). In addition, a multi-stub tuner requires to insert the bias tees (FIG. 1) between the DUT and tuner, which, in turn, reduces the tuning range, because the bias tees introduce additional insertion loss between the tuner and the DUT.
Further-on the shorted parallel stubs in such tuners do not have the benign “low-pass” behavior of the slide screw tuners, since they represent a high reflection (a “short circuit”) at high as well at low frequencies, causing many test devices to oscillate or even fail, in which case the testing becomes impossible.
What is needed, in fact, is a new impedance tuner apparatus, which takes advantage of the slide-screw tuner technology with its variable capacitive loads and the spacing between each load, which is the principle of the parallel-stub tuners.
In this document we propose a new apparatus (FIG. 5), which can operate at low radio frequencies in the range between 10 and 200 MHz; the new apparatus is a fully programmable electro-mechanical tuner, that can be remotely controlled by a control computer and can be calibrated using a vector network analyzer (VNA) and can be driven by a control software to synthesize user defined impedances at a frequency also defined by the user. The apparatus provides the capacity to be optimized for performance and tuning range in the various frequency bands it covers.
The new low frequency tuner consists of a cascade of at least three remotely adjustable variable capacitance blocks connected using lengths of low loss transmission lines, which can be rigid airlines or coaxial flexible or semi-rigid cables (FIG. 5).